1. Field of the Invention
This invention relates generally to a blue glass composition and to articles containing the blue glass.
2. Technical Considerations
A glass considered to be “spectrally selective” is one in which the light to solar heat gain ratio (LSG), which is defined as the total visible light transmittance (TVIS) divided by the solar heat gain coefficient (SHGC), is greater than or equal to 1.25. Examples of such glass are commercially available from PPG Industries, Inc. under the SOLEXIA®, CARIBIA®, AZURIA®, and ATLANTICA® family of glasses. A blue glass that is spectrally selective is very desirable, especially in the commercial trade market (i.e., architectural market).
Blue tint or color in conventional glass is achieved through the addition of one or more colorants. Perhaps the most widely used colorant for blue glass is cobalt oxide. Cobalt oxide is a very powerful blue colorant, with a color vector that is nearly completely in the negative b* direction (more blue) of the CIELAB color system. However, even a small amount of cobalt oxide added to float glass (such as 20 ppm or more) blocks enough visible light to render the glass to be non-spectrally selective.
Another method for making glass that is spectrally selective is by using greater than 0.4% iron oxide as the sole colorant while also adjusting the glass redox ratio to greater than 0.35. This combination gives the highest possible visible light transmittance attainable for the resulting low solar heat gain coefficient. For example, a glass containing 0.6 wt. % total iron and having a glass redox ratio of 0.45 would have an LSG ratio of about 1.35. However, as various other colorants are introduced to the system, the glass can become darker, i.e., the TVIS is reduced, and the LSG ratio can quickly decrease. For instance, adding as little as 20 ppm of cobalt oxide to the glass to try to give the glass a blue tint can lower the LSG ratio to about 1.24. Thus, the addition of cobalt oxide can have a negative impact on the LSG ratio.
Typically, a glass that is spectrally selective exhibits a green to greenish-blue color. In order to make a glass that has a purer blue hue (e.g., dominant wavelength range 480 nm to 495 nm at 6 mm thickness), a green-neutralizing colorant must be added to offset the green component that comes from the combination of Fe+2 and Fe+3 oxides. Selenium is most typically used for this purpose. The selenium color vector points towards the amber/orange direction. Although it is not directly opposite to the iron color vector, it acts to decolorize the green while adding a yellow component. However, the addition of selenium also has a strong negative impact on the LSG ratio. For example, the addition of just 4 ppm of selenium can lower the LSG ratio from 1.35 down to about 1.24.
Therefore, it is an object of this invention to provide a blue glass that is spectrally selective or non-spectrally selective.
A further object of this invention is to provide a blue glass that has a dominant wavelength in the range of 480 nm to 495 nm.
A still further object of this invention is to provide a blue glass that has desirable solar performance properties.
Another object of this invention is to provide a blue glass that has desirable visible light transmittance properties.